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Hunt the nectar

By Bob Holmes

WHEN honeybees draw the mental treasure maps they use to locate nectar-filled
flowers they have visited before, they can’t simply pace off the distance from
the hive. Instead, they use a visual mileometer that keeps track of the ground
they have covered.

“They are simply measuring how much the world whizzes by as they fly,” says
Mandyam Srinivasan, a vision researcher at the Australian National University in
Canberra.

Behavioural scientists have known for decades that honeybees measure the
distance to food sources, and signal this to their hive mates in the famous
“waggle dance”. Researchers once thought the bees measured distance by
monitoring their energy output, mainly because bees flying uphill to a food
source tend to signal a longer distance than those flying downhill. However,
more recent studies have contradicted this idea, leaving biologists unsure what
the honeybees use for a mileometer.

Srinivasan and his colleagues decided to put the matter to the test in a
glasshouse. First, they trained honeybees to find sugar water at the midpoint of
a transparent-topped tunnel 3.2 metres long, the inner walls of which bore
black-and-white stripes 2 centimetres wide. When the trained bees were later
sent into an identical tunnel that did not contain sugar water, the bees flew
back and forth in a series of diminishing U-turns that centred precisely on the
spot where their meal ought to have been.

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The bees continued their spot-on performance when the researchers used fans
to add a tailwind or headwind (Journal of Experimental Biology, vol
200, p 251). This would have made flying easier or harder, and should have
confused the bees if they were using energy expenditure or airspeed to measure
distance. It turned out that the bees were also unfazed when researchers changed
the number of stripes in the tunnel. This showed that they could not have been
counting landmarks to work out where they were.

But the team found that things changed dramatically when the walls of the
test tunnel bore horizontal stripes instead of vertical ones, depriving the bees
of any visual sense of motion. “When you remove the visual cues, they shoot
right through and bang into the end wall, then turn around and come back all the
way to the entrance, then try again,” says Srinivasan. “There’s no sign of any
hesitation near where the feeder should be.”

In further experiments, the researchers tested the bees in tunnels that were
narrower or wider than normal. Just as nearby scenery appears to go by much
faster in a car than distant mountains, a narrower tunnel should have given the
bees the illusion of moving farther and faster than a wide one.

Just as expected, in narrower tunnels the bees tended to undershoot their
target—that is, they thought they had flown farther than they actually
had. In wider ones, however, the team found that bees would overshoot.

The researchers say that the effect may also explain why bees perceive
distances differently flying uphill and downhill. Uphill bees fly closer to the
ground as it rises to meet them, while downhill bees tend to fly higher as the
ground drops away.